Various electronic systems convert alternating current (AC) power to direct current (DC) power in a multitude of applications. The AC power can be provided at numerous frequencies and at many voltage levels by a myriad of AC power sources. For example, an aircraft electronic system can be powered by a 120 volt (V) AC, a 60 Hz ground-based power supply, a 120 VAC, a 400 Hz aircraft engine alternator, or other AC power sources.
The 120 VAC, 400 Hz power is typically derived from alternators which are rotated by the engines of the aircraft. Accordingly, the frequency of the power provided by the alternators is often dependent on engine speed. In most conventional aircraft electrical systems, the alternators are coupled to a rectifier through constant frequency generators which ensure that the load is provided AC power at the same frequency from the alternators despite the operating speed of airplane engines.
Constant frequency generators are large electro-mechanical devices which can produce large amounts of heat. The large amounts of heat must often be dissipated by sizable dissipation devices, such as, oil cooling devices. Dissipating heat from constant frequency generators is particularly problematic because the constant frequency generators often must remove the most heat when the engine is running at its fastest speed, which is also when the engine generates maximum heat. For example, at take-off, the engine is often generating large amounts of heat as it is operated at maximum speed. The maximum speed associated with the engine also causes the alternators to generate maximum power, a portion of which must be dissipated as heat by the constant frequency generators.
The AC power from the constant frequency generator is typically converted to DC power and reconverted to AC power by switched converters, by power inverters, or is otherwise applied to an electrical load. Such well-known circuits usually include a large storage capacitor connected across a rectifier bridge. The large storage capacitor can cause the input current associated with the AC power to become highly non-sinusoidal. As shown in FIG. 1A, the non-sinusoidal nature of input current signal 6 results in a poor effective power factor for the AC power associated with current signal 6 and voltage signal 7. The poor power factor, which is manifested by current signal 6 lagging considerably behind voltage signal 7, requires the AC power source to provide a larger amount of power for a given power output.
With reference to FIG. 1B, the non-sinusoidal nature of current from the AC power source also can create high harmonic distortions. The high harmonic distortion can result in high peak currents, such as, a current signal 8 being drawn from the AC power source at a harmonic frequency. Relatively high current, such as, current signal 8, is drawn near peaks of a voltage signal 9 from the AC power source because conventional switched converters generally operate as a capacitive load for the received voltage from the AC power source. As a result, substantially zero current for the remainder of the cycle of current signal 8 and voltage signal 9 is drawn. This phenomenon results in a poor power factor and in a large harmonic distortion, which are manifested by drawing a larger root mean square (RMS) current from the power source when compared to a purely resistive load. For example, a conventional switched power supply can draw about 1.5 times the RMS current from the AC power source for a given power output as compared with a purely resistive load.
Commercially available products can be coupled between the AC power source and the switched power supply to reduce the power factor/harmonic distortion problems. Such products can cause a voltage signal 10 to be in phase with a current signal 11, thereby achieving an almost unity power factor, as shown in FIG. 1C. One such product, a boost converter, is disclosed in U.S. Pat. No. 4,677,366, issued to Wilkinson et al. on Jun. 30, 1997.
U.S. Pat. No. 4,677,366 describes a switched power supply which is well-known in the art. The switched power supply includes a boost converter coupled between a diode rectifier bridge and a storage capacitor of the switched power supply. The storage capacitor can be considered as part of the switched converter or as part of the boost converter. The boost converter is incorporated into the power supply because it draws a relatively smooth current from the AC power source (e.g., the line), and it permits the voltage on the storage capacitor to be higher than the voltage produced by the diode rectifier bridge, thereby providing more efficient energy storage.
The boost converter includes an inductance coil, a transistor, a diode, and a capacitor. The inductance coil is coupled in series with the diode. The capacitor is coupled between the cathode of the diode and ground, and the transistor is coupled between the anode of the diode and ground (between the inductor and the diode).
The boost converter is configured to draw a sinusoidal current by operating as a current regulator with a current reference control signal set to track the voltage from the power source. The current reference control signal controls the voltage across the capacitor. The voltage across the capacitor is regulated by controlling the magnitude of the current through the boost converter by modulating the transistor. In this way, the boost converter allows the switched converter to achieve a unity power factor for a known, constant AC power input signal.
Nonetheless, commercially available products, such as, the switched power supply including the boost converter disclosed in U.S. Pat. No. 4,677,366, exhibit poor input step response performance over wide input frequency ranges, such as, the operating ranges of 50 Hz-1000 Hz associated with aircraft AC power supplies. For example, an aircraft power source can change from 120 VAC, 60 Hz when ground-based, to 120 VAC, 400 Hz when flying. Additionally, aircraft power supplies can be variable frequency power sources which develop power at voltage levels and at frequencies dependent on engine speeds. Therefore, conventional switched power supplies including boost converters cannot adequately handle variable AC power, especially in aircraft applications.
Thus, there is a need for a power supply which has optimized power factor and harmonic correction operation. Further still, there is a need for a power supply which is operational over a large frequency range. Even further still, there is a need for an aircraft power supply which does not require a constant frequency generator.